Stimulus response technique(SRT): Stimulus response technique is used for measuring residence time distribution (RTD) of liquid phase.
Stimulus means something that cause activity (Tracer in a reactor). so if we introduce some colored material(tracer) in a reactor, how the colored material distributed/responded within the flowing fluid inside the reactor from starting in reactor at time t=0 to exit of the reactor time t=t.
The exact definition/meaning of SRT is, to estimate the RTD, the ideal situation is to measure or obtain in information on each and every molecule (concentration, Temperature, velocity) so that one can calculate conversion in the reactor. this is impossible at molecular level. hence we should resort to at least getting information on a group of molecules using tracer technique are more preciously stimulus response technique.